Various aspects of the biosynthetic pathways which culminate in phosphatidylcholine and phosphatidylethanolamine have been investigated in Lemna. Formation of ethanolamine occurs via decarboxylation of a water soluble compound (probably serine, although phosphoserine has not been completely eliminated as a dandidate). Methylation occurs almost entirely at the phosphobase level. These processes are stringently controlled by, respectively, ethanolamine and choline, which are taken up from the external medium via specific and avid active transport mechanisms.